1. Field of the Invention
The present invention relates to digital data encoders and decoders and more particularly to a combined encoder/decoder (transcoder) for conversions between pulse code modulation and delta modulation.
The time shared digital coder/decoder disclosed herein is used to convert between different sampled data signal formats. Numerous formats are in use today. Those of interest here belong to a family of digital transmission rates of 8000 n b/s wherein n is any integer. These rates apply universally to telecommunications equipment used commerically and in the military in the United States and Europe. The rate family is based on the accepted sampling rate of 8 KHz for 3 KHz bandwidth speech waveforms. The integer n typically indicates the number of bits used to encode the speech samples. For most cases n = 5, 6, 7 or 8 bits per channel. We will refer to systems which transmit codewords of n bits per word at 8000 words per second per channel as PCM systems.
2. Description of the Prior Art
Wideband communications links have been established to carry many channels of these digitized signals. Because of the established links, it is anticipated that future systems will use the same 8000 n family of rates. Formats may be expected to change as more efficient encoders/decoders become available.
One new format that is gaining acceptance in the military, and at a slower pace commercially, is delta modulation, including continuously variable slope delta modulation (CVSD). This format provides for rates corresponding to n = 2, 3, or 4 bits per channel.
Delta modulation uses 1 bit codewords transmitted at a higher sampling rate of 16 Kbps to 32 Kbps. It is ideally suited to serial transmission and does not require serial-to-parallel converters and complex synchronizers common to contemporary PCM equipment. (PCM equipment must convert between a serial transmission format and distinct codewords of typically 5, 6, 7 or 8 bits each.)